The present invention relates to seismic acquisition apparatuses.
In conventional practice, one or more detectors are used to perform seismic acquisition, each detector being constituted by a group of elementary sensors (geophones) distributed on the ground and interconnected so as to deliver the analog output of the detector to which they belong.
The analog signals output by the various detectors are seismic traces which are generally digitized prior to being recorded for subsequent processing.
Usually, the length of a detector is selected so as to achieve a compromise between eliminating seismic noise at long wavelengths and preserving high frequencies of the signal.
It is possible to eliminate surface noise by using groups of sensors of a length that is greater than the longest wavelength of the expected noise, but in so doing, the high frequencies of the reflected signal that are to be recorded are also attenuated.
To avoid such a compromise, proposals have been made in GB-1 391 829 for seismic acquisition apparatus in which the sensors within the same group are subdivided into a plurality of subgroups, each corresponding to a band of spatial frequencies, the analog signals output from the various subgroups being time-filtered, and then recombined to deliver the seismic trace corresponding to the group of sensors.
Thus, upstream from the recording, a fan of space-time filtering is performed enabling seismic traces to be acquired that are cleared of the long wavelengths of seismic noise but that maintain the high spatial frequencies of the signal as received at the elementary sensors.
In a more recent variant of that technique, as described in U.S. Pat. No. 4,499,565, it is proposed to use sensors having different natural frequencies for the various subgroups, such that the effect of combining the sensors is to perform both space filtering and frequency filtering directly.
Nevertheless, even with such prior space-time filtering, the seismic traces recorded at the outlet from such apparatuses are not of satisfactory quality.
The main object of the invention is to improve the quality of the seismic traces that are recorded.
Already disclosed in U.S. Pat. No. 5,550,787 is seismic acquisition apparatus in which the signals output by the detectors are acquired by means of an acquisition system including, in particular, a lowpass filter, a preamplifier, and a highpass filter, after which the signals are processed for storage purposes.
The invention provides a seismic acquisition apparatus comprising a plurality of detectors constituted by one or more seismic sensors, space-time filtering means which generate at least one seismic trace corresponding to a group of detectors by combining signals output therefrom, and storage means for storing the seismic traces generated in this way, the apparatus being characterized in that it comprises seismic preprocessor means for re-phasing the signals output by the detectors and/or for space-time filtering said signals prior to the signals being summed.
Advantageously, the preprocessor means also comprise means for determining at least one parameter representative of the quality of the signals picked up by the detectors.
Preferably, the preprocessor means are of the digital type.
Also, at least one detector belongs to two different groups, the preprocessor means using the signal output by said detector to generate seismic traces corresponding to both of said groups.
It will be observed that the connections used in the acquisition apparatuses described both in GB 1 391 829 and U.S. Pat. No. 4,499,565 are complicated and expensive, requiring a large number of elementary sensors. Consequently, they are difficult to implement.
Digital type preprocessor means make implementation very simple. They also make it possible to use a single detector in a plurality of groups, and consequently to reduce significantly the number of detectors and thus the cost of the acquisition apparatus.
Furthermore, in the techniques proposed in GB 1 391 829 and U.S. Pat. No. 4,499,565, the filtering performed remains imprecise and is no more than a mere staircase approximation to ideal filtering corresponding to a constant apparent speed. In addition, those filters suffer from the drawback of being xe2x80x9cfrozenxe2x80x9d by construction and of being difficult to modify during acquisition.
Another object of the invention is thus to propose acquisition apparatus that makes it possible to perform space-time filtering of better quality than that made possible by the apparatuses described in GB 1 391 829 and U.S. Pat. No. 4,499,565, and also to provide greater flexibility in processing.
To this end, the preprocessor means perform space-time filtering of the progressive fan type on the output signals from the detectors.
Filtering is said to be xe2x80x9cprogressivexe2x80x9d when it corresponds, in a representation in the space-frequency domain, to attenuation level curves that are continuous.
It will be observed that the use of digital preprocessor means for combining the signals output from the detectors and for forming the seismic traces makes it possible to perform such progressive filtering, and thus to perform filtering that coincides with the desired ideal filtering.